A controllable RF phase shifter should ideally have minimum size, weight, cost and complexity along with low insertion loss, low insertion loss modulation, temperature stability, low drive power requirements and the ability to obtain a desired phase shift in a fast and accurate manner. Although improvements continue to be obtained in the art, still further improvements are required for many applications.
Radar applications having high pulse repetition frequencies in the 200 to 300 kHz range require relatively small high performance controllable reciprocal phase shifters operable in the upper frequency ranges. For example, radar applications involving planar phased arrays for use in aircraft require high performance phase shifters in the X- and K-bands.
Existing planar substrate diode phase shifters may be used in such applications and offer the advantage of being reciprocal between transmit and receive modes, thus eliminating the necessity for switching. The use of reciprocal phase shifters for the above-noted applications is considered a must in most instances due to the aforementioned high pulse repetition frequency required. In this regard if a non-reciprocal phase shifter is used, it must be switched in order to obtain reciprocity between transmit and receive modes which would require the phase shifter to switch at a rate twice the pulse repetition frequency, 400-600 KHz for most airborne moving target indication (MTI) modes. On the other hand, the use of reciprocal phase shifters that are switched only when the beam position for a phased array is changed may require switching at a much lower rate of only about 500 Hz.
In phased array applications of the aforementioned nature insertion loss, insertion loss modulation, RF power, bandwidth, phase accuracy, switching time, switching power and, of course, cost are critical. A waveguide mode twin slab ferrite phase shifter of the nature described in commonly assigned U.S. Pat. No. 4,445,098 to Sharon et al excels in some of these areas. Such twin slab phase shifters, however, are typically mounted in a waveguide housing which is not compatible with microstrip. Accordingly, such phase shifters are relatively large and expensive. Moreover, they are non-reciprocal and if unswitched reciprocity is desirable, these elements must be used in conjunction with circulators, thus further increasing the size.
As may be seen from a review of the above noted copending related application to Roberts et al, Ser. No. 07/330,617, filed Mar. 30, 1989, now U.S. Pat. No. 5,075,648 issued on Dec. 24, 1991, the Sharon et al type of dual toroid ferrite phase shifter may be greatly miniaturized and incorporated serially with a microstrip transmission line to produce a very small, essentially planar phase shifter which excels with respect to most of the above noted critical parameters of a phased shifter for a phase array application. Such hybrid phase shifters, however, are also non-reciprocal.
I have discovered that by properly combining diode and ferrite phase shifter technology a planar substrate ferrite/diode phase shifter for phased array applications is obtainable which offers significant advantages over ferrite technology and major improvements over existing planar substrate diode phase shifter technology.
In a nutshell such advantages may be obtained by using ferrite technology for a 180.degree. controllable phase shifter stage with the remaining controllable phase shifter stages employing diode phase shifters for a composite controllable phase shifter especially usable in a phased array application. Although the ferrite 180.degree. stage is non-reciprocal, the 180.degree. stage does not require switching between transmit and receive modes in a typical phased array application. That is to say, the ferrite 180.degree. stage in phased array applications does not require switching between transmit and receive modes since the 180.degree. offset is of no consequence in many scanning arrays. The remaining stages, however, are required to be reciprocal to avoid switching between modes. Diode technology may be used for these remaining stages since they are inherently reciprocal.
The use of PIN diodes in phase shifting arrangements for use in phased arrays is well known as indicated by: the PIN Diode Designers' Handbook and Catalog by Unitrode Corp., Lexington, Mass., pages 99 through 101; and "A Diode Phase Shifter for Array Antennas", by J. F. White, 1964 PTGMTT International Symposium Program and Digest, pages 181 through 185.
Exemplary embodiments combining diode and ferrite technology in the manner detailed below offers several significant advantages over an all diode phase shifter. Moreover, such advantages are even more significant at the higher microwave frequencies. For example, the disclosed exemplary embodiment exhibits a lower insertion loss and insertion loss modulation, lower VSWR and VSWR modulation along with higher power handling capability along with lower drive power. Moreover, there are advantages compared to other competing technologies, such as the dual mode reciprocal ferrite phase shifter and the hybrid mode reciprocal ferrite phase shifter (where reciprocity is obtained by pairing two such phase shifters or by switching). Although the advantages are somewhat less in magnitude compared to those pertaining to diode phase shifters, the advantages of my exemplary embodiments are nevertheless significant. For example, distinct advantages are obtained with respect to temperature stability, switching speed and in many applications, lower cost as well.